What Is Diphenhydramine HCL?

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  • Written By: Clara Kedrek
  • Edited By: Jessica Seminara
  • Last Modified Date: 21 March 2018
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Diphenhydramine hydrochloride (HCL) is an antihistamine medication used to treat allergic conditions, insomnia, and motion sickness. It works by blocking the action of histamine, a chemical compound found in the body. Although diphenhydramine HCL is very effective in stopping allergic reactions, its use to treat these conditions has decreased over the years with the introduction of newer medications that have fewer side effects. Common adverse reactions associated with the drug include dry mouth, sedation, urinary retention, blurry vision, and constipation.

In the body, diphenhydramine HCL opposes the action of histamine, a chemical compound that is important in causing allergic reactions, stimulating the production of acid in the stomach, and increasing alertness. The antihistamine medications counteract histamine's effects. Diphenhydramine HCL goes by the brand names Benadryl®, Aler-dryl®, Unisom®, and Allermax®. It is often referred to as simply diphenhydramine because hydrochloride is an inactive stabilizing agent. The medication is available over the counter or by prescription.

Diphenhydramine was one of the first agents in its class to be discovered, and is considered to be a first-generation antihistamine. The medication has been used since the 1940s to treat allergic conditions such as seasonal allergies, hay fever, and acute allergic reactions to food or medications. Additionally, it has been used to treat the common cold because it can decrease nasal secretions.


Over the years, new types of antihistamines have been introduced, including the second- and third-generation antihistamines. Some of these newer medications, such as the drugs cetirizine and loratadine, have become increasingly popular treatments for allergic conditions. Many people prefer them because they do not cause the sedation associated with diphenhydramine HCL and other first-generation antihistamines.

A number of clinical applications for diphenhydramine HCL remain relevant. Since it is very effective in fighting allergy symptoms, some people continue to use it in this capacity despite its side effects. Other people take advantage of the sedating effect this medication has and use the drug as a sleep aid. Additionally, many people use the medication for motion sickness, as it can decrease the nausea associated with this condition.

Side effects of diphenhydramine commonly include drowsiness, dizziness, dry mouth, headache, constipation, and elevated heart rate. Although the medication is well known for its sedating effects, some children who take it can have a paradoxical increase in their alertness. Occasionally, the medication has been known to cause deadly heart arrhythmias, or abnormal rhythms, in people with underlying heart disease.

Not all people are good candidates for treatment with diphenhydramine. It should be used with caution in elderly patients because it can cause delirium. Patients with asthma, gastrointestinal obstruction, benign prostatic hypertrophy, and glaucoma could experience more severe side effects, and these people should only use the medication under the supervision of their doctors.



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